QUESTION:
Does HST occupy a Lagrange point in space?

ANSWER from Dick Shaw on April 3,1996:
By "Lagrange point" I suspect you are referring to the 5 points of quasi-
equilibrium in the Earth-Moon system where the orbital period of an object
placed there would match that of the lunar orbit around the Earth.

The short answer is no, HST does not occupy a Lagrange point, but rather
operates in a low Earth orbit some 320 nautical miles (or not quite 600
kilometers) above the Earth's surface.  It is easy to see why: the HST mission
plan calls for periodic servicing of HST to carry out minor repairs and
to replace some of the science instruments every few years with new, more
capable ones.  The cheapest way to perform these servicing missions is to
use the Space Shuttle to ferry the new instruments and a repair crew to HST.
The Shuttle (or STS, in NASA parlance, for Space Transportation System)
can only reach orbits of (roughly) less than 400 miles.  As you probably
know, the Lagrange points are quite far from the Earth -- in fact they lie
a fair fraction of the distance to the Moon, which is roughly 238,000 miles
(380,000 kilometers) away.  So the Lagrange points were not an option for
the HST orbit.

Since you asked this question, perhaps you have already thought of some good
reasons why NASA might want to put an astronomical satellite in orbit at a
Lagrange point.  For instance, it would greatly increase observing efficiency
because the bright Earth would not get in the way of observations for a large
part of every orbit, as happens now with HST.  Perhaps you can think of other
reasons as well.

You may be interested to know that NASA recently began a study to come up
with a design concept for the Next Generation Space Telescope (NGST).  This
telescope could be 2 to 4 times the diameter of HST but cost much less.  If
approved by Congress, it could be launched in the middle of the next decade.
One possibility being considered is to put NGST at a Lagrange point in the
Earth-Sun system which lies in the direction opposite the Sun.  This would
put NGST in a cold, dark environment which is quite favorable for
astronomical observations.